Characteristics of Medial Depression of the Mandibular Ramus: A CBCT Analysis in Different Sagittal Skeletal Patterns

Statement of the Problem: Medial depression of the mandibular ramus (MDMR) as a normal anatomical variation might complicate orthognatic surgeries that involve ramus. When planning an orthognatic surgery, it is clinically valuable to notice MDMR in osteotomy site to decrease the risk of failure. Purpose: The aim of present study was to evaluate the prevalence as well as characteristics of MDMR in three skeletal sagittal classifications. Materials and Method: This cross sectional study evaluated 530 cone beam computed tomography (CBCT) scans, of which 220 were enrolled. The skeletal sagittal classification, the presence of MDMR, the shape, depth, and width of MDMR were recorded for each patient by two examiners. Chi-square test was performed to determine the differences between three skeletal sagittal groups and between two genders. Results: The overall prevalence of MDMR was 60.45%. MDMR was mostly detected in class III (76.92%), followed by class II (76.66%), and class I (54.87%). In the analyzed CBCT scans, semi-lunar was the most common shape detected (42.85%), followed by triangular (30.82%), circular (18.04%), and tear-drop (8.27%). The depth of MDMR was not significantly different between three sagittal groups and between genders; however, the width of MDMR was higher in class III group and in male patients. In the present study, MDMR was found to be more common in patients with class II and class III skeletal classifications. Although, MDMR was more frequent in class III, the difference between class II and class III was not significant. Conclusion: More caution is needed during orthognatic surgery in patients with dentoskeletal deformities during the splitting of the ramus. Moreover, higher width of MDMR in class III and male patients should be concerned when planning an orthognatic surgery for these patients.


Introduction
Medial depression of the mandibular ramus (MDMR) or medial sigmoid depression is a normal anatomical variation first reported by Langlais et al. [1]. This depression is located just below and slightly anterior to the most inferior aspect of the sigmoid notch ( Figure 1) [1][2].
This area appears as a radiolucent foramen because of a decrease in X-ray absorption and therefore, it might be misinterpreted as a pathological entity [3][4]. It has been reported that MDMR might complicate the splitting of ramus during orthognatic surgery due to the fusion of the medial and lateral cortical plates in patients with dentoskeletal deformities [5]. On the other hand, it is reported that this depression is associated with high mu- Figure 1: Medial depression of the mandibular ramus (MDMR) was shown by black line [7] scle activity, which can increase the potential relapse in orthognatic surgery [6].
Ethnical and congenital factors are assumed to affect the prevalence of MDMR as reported in different ethnic population [7]. The prevalence of MDMR is ranged mostly from 5.3 to 32.7% in previous studies [1,[7][8][9]; however, the prevalence of MDMR was reported 70% in an Indian population [10]. In addition, previous studies declared the higher prevalence of MDMR in dentoskeletal deformities using panoramic radiographs [7][8][9][10][11]. Additionally, the use of cadavers and dry skulls, concerning the difficulty in prediction of age and gender, are not at ease for studies [5,12].
Considering the limitations of the panoramic radiographs and dry mandibles and regarding the importance of MDMR in selection of osteotomy site with the least risk of fracture in orthognatic surgery, especially in sagittal split osteotomy and gross bone resection in mandibular corpus malignancies, the current study was designed to use CBCT scans for determining the prevalence and the characteristics of MDMR in patients with different sagittal skeletal classifications.

Materials and Method
A total of 530 CBCT full-face scans of patients referred to Oral and Maxillofacial Radiology Department of Shiraz Dental School were examined. Informed consent was obtained for experimentation with human subjects.
The privacy rights of human subjects were observed.   assessed. In addition, the relationships between the shape, the depth, and the width of MDMR and the gender of the patients were evaluated. All CBCT images were analyzed with NNT software. Chi-square test was performed to analyze the results and to determine the differences between three skeletal sagittal classifications and between genders. The data were analyzed using SPSS (Statistical Package for Social Studies) version 23.00. The statistical significance was set at p˂ 0.05.

Results
The overall prevalence of MDMR was 60.45%. A total of 133 out of 220 CBCT scans showed MDMR (either unilateral or bilateral). MDMR was most frequently detected in class III patients (76.92%), followed by class II (76.66%), and class I patients (54.87%) ( Table 1).
Although MDMR was more frequent in class III, the difference between class II and class III was not statisti-cally significant. In CBCT scans detected with MDMR, semi-lunar was the most prevalent shape identified in 42.85% of patients, followed by triangular (30.82%), circular (18.04%), and tear-drop (8.27%). The most prevalent shape in class I patients was semi-lunar, followed by triangular, circular, and tear-drop. In class II patients, both semi-lunar and circular were the most prevalent shapes, while depressions with triangular and tear-drop shapes were less common. In Class III patients, triangular presented the highest prevalence; followed by semi-lunar and circular, while, tear-drop shaped depressions were not seen in this group ( Table   2). The depth of MDMR did not show a statistically significant difference in three skeletal sagittal classifications; whereas, the width of MDMR was greater in class III compared to class I and class II (p Value: 0.003) ( Table 2). The depth of MDMR did not differ between male and female patients; the mean depth of MDMR   ).

Discussion
The results showed that MDMR was most frequently detected in class III (76.92%), followed by class II (76.66%), and class I (54.87%). In the analyzed CBCT scans, semi-lunar was the most prevalent shape, followed by triangular, circular, and tear-drop. Although the depth of MDMR was not significantly different between three sagittal groups and between genders, the width of MDMR was higher in class III group and in male patients.
Panoramic view is a two-dimensional image and only the structures that fall within the focal trough can be trusted. In addition, the airway shadow, the pterygoid plates, the soft palate, and other structures superimposed on the sigmoid notch region might lead to misinterpretation [2]. Besides, the subjectivity of interpreting panoramic radiographs must be considered [7,10]. The limitations of the panoramic radiographs might be responsible for the differences between the prevalence of MDMR reported in mandibular specimens and in patients' radiographs as well [7,[9][10].
Muto et al. [12] and Yu et al. [5] have criticized the use of cadavers and dry skulls in anatomic studies because this type of material does not provide data regarding the age and the gender of the sample. Moreover, the morphology of dry skulls is usually very different from the young patients who usually undergo the correction of dentoskeletal deformities [11][12].
The prevalence of MDMR ranged from 5.3% to 32.
7% in previous studies, which were mostly conducted on panoramic radiographs [1-2, 7-9] and only one study was performed using CT scans [13]. However, in Asdullah et al. study [10], the prevalence of MDMR was 70% in panoramic radiographs in Indian population. In the present study, the overall prevalence of MDMR in patients' CBCT radiographs was 60.45%. The discrepancy in the results of the current study and the previous studies might be due to the different methods used. To avoid the distortion, unequal magnification, and the superimposition of adjacent structures as the main drawbacks of using panoramic radiographs, we employed CBCT images. In addition, ethnic variability was observed among different studies.
The results of the current study showed that MDMR was mostly detected in class III, followed by class II, and class I. In a study conducted by Carvalho et al. [7] who compared the prevalence of MDMR in patients with dentoskeletal deformities and class I group, higher prevalence of MDMR was found in cases with dentoskeletal deformities. Dalili et al. [8] and Sudhakar et al. [9] reports indicated that MDMR was more prevalent in class II and class III groups, although the differences reported in the present study were not statistically significant. The results of the present study and the before mentioned studies suggest examining patients to identify MDMR prior to orthognatic surgery to avoid undesirable outcomes.
Carvalho et al. [7] found that triangular shape was the most prevalent one of MDMR, followed by semilunar, tear-drop, and circular. Sudhakar et al. [9] and Asdullah et al. [10] reported higher prevalence of semilunar in their studies, followed by triangular, which is similar to our study. However, in these studies, circular was the least common shape [9][10]. Sudhakar et al. [9] found higher prevalence of semi-lunar in all skeletal classifications. We found the same results in class I and class II groups; however, in our study triangular was the most prevalent shape in class III group.  [6]. Because of the functional adaptation in the ramus in response to the insertion of medial and posterior attachments of temporal muscle to this area, functional patterns and bite forces play noticeable roles in determining the characteristics of MDMR [6,[14][15].
We found no significant differences in MDMR depth between three skeletal sagittal classifications and between male and female patients. Our results might sup-port the idea that there is no difference in craniomandibular muscle activity in different sagittal skeletal disharmonies.
Further studies are recommended to illuminate the exact association between MDMR characteristics and muscle function by using CBCT and other advanced modalities.

Conclusion
MDMR was more prevalent in patients with class II and class III skeletal classifications. Although, MDMR was more frequent in class III, the difference between class II and class III was not statistically significant. Therefore, more caution should be regarded in patients with dentoskeletal deformities during the splitting of the ramus. On the other hand, the higher width of MDMR in class III and male patients should be concerned when planning an orthognatic surgery for these patients.